An optical delay line, or buffer, is a key element of the future photonic circuits for optical signal processing in applications such as communications and computing. Conventionally, an optical delay line consisted of one or more sections of optical fiber, where the length of the fiber determined the delay introduced into the propagating signal. For example, a standard optical fiber having a length of about 20 meters (m) will introduce a 100 nanosecond (ns) delay to a pulse of light. To provide compact packaging of this type of delay device, the 20 m of fiber would be coiled to fit into a relatively small package. The ultimate size of the package is limited, at least in one sense, by the bend loss of the optical fiber, which increases with decreasing radius of the coil. For a 20 m length of fiber, for example, a package in the form of a box having dimensions of several cubic centimeters is required to minimize the impact of bend loss. With the on-going efforts to miniaturize optical components, these dimensions are becoming problematic.
In contrast to fiber-based delay lines, integrated photonic components such as microsphere or microtoroid resonators have been demonstrated to provide a similar amount of delay (i.e., hundreds of nanoseconds) in component sizes ranging from several tens of microns to several millimeters. While the amount of introduced delay falls within the desired range, the delay time/bandwidth product limitation of these microresonators restricts the corresponding bandwidths to pulses of about 1 MHz only (i.e., hundreds of ns delay and a bandwidth of only 0.00001 nanometer (nm)). This bandwidth is far too small for these microstructured resonators to be considered as a realistic optical buffer for commercial systems.
Thus, a need remains for an optical delay element that has a larger bandwidth than the known microresonator devices, yet is more compact than conventional fiber delay lines without introducing loss or reliability issues.